Microwave transition device



Apr-1] 25, 1961 J. s. AJlOKA ET AL MICROWAVE TRANSITION DEVICE 2 Sheets-Sheet 1 Filed Jan. 6, 1959 INVENTORS, JAMES S. AJIOKA, BERNARD l WALSH,J:,

3 Em 3 mi Mm NE) April 1961 J. s. AJIOKA ET AL 2,981,904

MICROWAVE TRANSITION DEVICE Filed Jan. 6, 1959 2 Sheets-Sheet 2 l/VVE/VTORS.

JAMES S. AJIOKA, BERNARD L. WALSH, Jr.,

ATTORNEY.

, 2,981,904 MICROWAVE TRANSITION DEVICE James S. Ajioka, Torrance, and Bernard L. Walsh, Jra, Granada Hills, Califl, assignor's to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Jan. 6, 1959, Ser. No. 785,207

3 Claims. Cl. 333-44 This invention relates to microwave devices, and particularly to devices forproviding a transition between different waveguide sections which are colinear with each other;

It is often useful to employ different microwave waveguides in the same system, in order to utilize structural or electrical properties of the individual types of waveguides. For example, it is often desirable to use rectangular waveguide and'coaxial waveguide. These guides have different characteristic impedances and also propagate energy in dominant modes, and each may have particular utility for different circumstances.

It is therefore often desirable to provide transitions betweenthe' different types of waveguide. which are employed in a system. Similarly it is desirable in many instances to provide transformations between different modes. The devices heretofore available, however, have usually been subject to a number of disadvantages. For one thing, these devices have often involved excessive insertion loss and reflection. Another factor is the fact that the devices haveoften been inherently narrow band and not suitable for; applications in which frequency sensitivity could not be tolerated. These disadvantages have particularly been, encountered with configurations which are intended to provide a transition. between two waveguide sections which lie along the same axis. The problems involvedin providing a suitable transition for the energy and art-effective transformation of the mode have in these devices usually resulted in cumbersome or complex structures.

A transition device which would make possible a simple colinear transition between two waveguidescctions wouldpermit the provision of microwave devices having useful properties. Arotary' member, for example, could then be coupled, to a stationary member by a rotary joint of simple configuration. i

It is therefore an object of the present invention to provide an improved micro-wave transition device for coupling-energy between colinear microwave waveguides of different geometries.

It is another object of this invention to provide an improved microwave transition coupling which operates along a given axis and effectively transforms energy from onemode to another with low reflection over a broad frequency band.

Another object of this invention is to provide an improved microwave transition coupling for coupling 'energy between a rectangular waveguide and a coaxial waveguide which lies along the same central axis;

These and other objects of the present invention are achieved by a microwave transition device which may,

for example, couple energy between a rectangular and a coaxial waveguide, both vof which waveguides lie along a common central axis. The rectangular waveguide may include three ridge segments, one of which is centrally positioned on the inner side of one of the broad walls, and the other two of which are positioned against the other'bro'ad wall, and all three of which have successively Patented Apr. 25,- 1961 2 increasing height in the direction toward the coaxial waveguide. These ridge segments may reach a maximum height within the rectangular wavegiude of approximately half the waveguide height, with the centrally positioned ridge segment terminating in a protruding. portion which contacts the center conductor of. the coaxial waveguide. These ridge elements provide a gradual mode distortion between the T15 mode of the rectangular waveguide and the dominant TEM mode of the coaxial provides a structure which gradually enccompasses the v waveguide. vThe transition is completed by a unitary conductive block portion partially within rectangular waveguide and joining the rectangular waveguide to the coaxial waveguide. This unitary conductive block has a central aperture registering with the inner surface of the outer conductor of the coaxial waveguide, but the block is itself tapered. A planar tapering of the block 'center conductor of the coaxial line, in the direction toward the coaxial waveguide, and thus gradually com.- pletes the transformation to the dominant mode of the coaxial waveguide. p f

The arrangement thus provided is simply constructed but provides a transition along a single axis whichis broad-banded and has excellent impedance matching characteristics. Such an arrangement may be used to provide a rotary joint through the use of a rotating section of coaxial waveguide coupled to the coaxialportion of the transition devices. 7

In accordance with another feature of this invention, conductive and dielectric transition sections may be used in combination. The triple ridge elements described above may be coupled to; a dielectric element of gradually increasing cross section within the coaxial waveguide. The concentration of energy provided by the dielectric element performs the function of. completing the mode transformation in smooth fashion. I

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, may bestbe understood when considered in the light of the following description, when taken'in connection with the accompanying drawing, in which like reference numerals refer to like parts, and in which:

Fig. 1 is a. perspective view, partially brokenv away; of a microwave transition device in: accordance with the present invention;

Figs. la and 1b are respresentations of the electric field distributions of rectangular waveguide and coaxial waveguide.

Fig. 2 is aside section. elevation view of the device of Fig. 1; T

Fig. 3 is a perspective view, partially'brokenaway, of a second form of microwave transition device in accordance with the invention;

Fig. 4 is a diagrammatic representation of successive cross sections of the transition device of Fig. 1, showing the electric field configurations at various points therein;

Figs. 5 and 6 are views of segments of rectangular and coaxial waveguide, respectively, showing the similarity; between certain magnetic field configurations;

Fig. 7 is a sectional side elevation view of another arrangement in accordance with the invention, showing the combined use of. conductive and dielectric transition se'c= tions, and

Fig. 8 is a diagrammatic representation, in simplifiedform, of the cross-sectional configurations at different. points along the length of the arrangement of Fig. 7.

An arrangement in accordance with the invention, referring now to Figs. 1 and 2, may provide energy coupling and mode transformation between a rectangular wavetribution' in a rectangular waveguide, the dominant TE jwall 12, and narrow walls 14 and 15.

mode in the rectangular waveguide has electric field cominvention.

The arrangement of Figs. 1 and 2 includes a rectangular waveguide having broad walls 11 and 12 and, hereafter referred to as the first broad wall 11 and the second broad The rectangular waveguide 10 has a central longitudinal axis, and terminates in this instance at an open end to which is coupled a colinear coaxial waveguide 20 having an outer conductor 21 and a centrally disposed inner conductor 22. The end of the coaxial waveguide section 20 which is spaced apart from the point of joinder of the coaxial waveguide 20 and the rectangular waveguide 10 may be threaded, as shown, to couple to additional coaxial waveguide sections (not shown). Similarly, the free end of the rectangular waveguide 10 may be coupled by flanges or other means (not shown) to associated rectangular waveguides or other microwave components (likewise not shown).

The central conductor 22 of the coaxial waveguide section 20 extends partially into the rectangular waveguide section 10. The central conductor 22 is coupled to a tapered ridge element 31 which is afiixed to the first broad wall 11 of the rectangular waveguide 10. The tapered ridge element 31 forms a portion of a transition section 30 which lies at least partially within the rectangular waveguide 10 and which electrically and mechanically couples the rectanguar waveguide 10 to the coaxial waveguide 20. The first ridge element 31 has a successively increasing height within the rectangular waveguide 10 in the direction toward the coaxial waveguide section 20 and terminates in a protruding finger or faired section 32 to which is attached the center conductor 22.

Thus the first ridge element 31 changes from a section of minimum height adjacent the free end of the rectangular waveguide 10 to a section 32 which merges with the center conductor 22 in the coaxial waveguide section 20. At the point of joinder, the first ridge element 31 and the center conductor 22 need not be of like cross section, but should not have a marked disparity in size or configuration. i The first ridge element 31 therefore provides what may be considered to be a central transition element or inner conductor transition. Outer transition elements are provided in the arrangement of Figs. 1 and 2 by a like pair of fins or ridge elements 34, 35 positioned against the second broad wall of the rectangular waveguide 10. The second and third ridge elements 34, 35 are symmetrically positioned between the centerline of the rectangular waveguide 10 and a different narrow wall, 14 or 15. The secend and thitrd ridge elements 34 and 35 have a progressively increasing height in the direction toward the coaxial waveguide section 20, and thus have an opposite taper to the first ridge element 31. All of the ridge elements 31, 34 and 35 terminate, on the side of the coaxial waveguide section 20, in approximately the same plane normal to the walls of the rectangular waveguide 10. The ridge elements 31, 34 and 35 extend approximately half the height of the rectangular waveguide 10.

The remaining transition between the wave confining structure provided by the three ridge elements 31, 34 and 35 and the rectangular waveguide 10 is provided by a unitary conductive block 38 positioned within the rectangular waveguide 10 at the transition section end. The

unitary conductive block 38 has a central aperture 39 which substantially matches and registers with the inner surface of the outer conductor 21 of the coaxial waveguide section 20. The innermost surface of the unitary conductive block 38 abuts against the highest portion of the second and third ridge elements 34 and 35. The block also is provided with a planar surface 40 which tapers toward the first broad wall 11 of the rectangular waveguide 10 from the highest portions of the second and third ridge elements 34 and 35.

It should be noted that the first ridge element 31 has, at the transition section 30, a configuration which is substantially symmetrical with the central axis of the rectangular waveguide 10. The second and third ridge elements 34 and 35 at the corresponding point are approximately half the height of the rectangular waveguide 10 and join smoothly to the lowest edge of the tapered surface 40 of the unitary conductive block 38. The tapered surface 40 and the central aperture 39 of the unitary conductive block 38 thus define a transition aperture which may be considered gradually to encompass the central conductor 22 of the coaxial waveguide section 20. Along the central conductor 22 in the direction toward the coaxial waveguide section 20, the cross section of the unitary block 38 changes gradually from a hemispherical shape to a concentric fully encompassing surface corresponding in shape and axial position to the outer conductor 21 of the coaxial waveguide section 20.

The operation of this arrangement in providing a gradual transition of desirable electrical characteristics between the TB mode of rectangular waveguide (see Fig. 1A) and the TEM made of coaxial waveguide (see Fig. 13) will now be apparent by reference to Fig. 4 as well as Figs. 1 and 2. The three ridge elements 31, 34 and 35 within the rectangular waveguide 10 provide a gradual transformation of the mode within the rectangular waveguide 10. It is assumed here for purposes of description that energy transmitted along the rectangular waveguide 10 is to be transmitted to the coaxial waveguide section 20, but the same considerations apply and the same results are obtained when the correction of transmission is the opposite. As may be seen in Fig. 4B, the presence of the ridge elements 31, 34 and 35 causes a gradual distortion of the electric field which results in a concentration about the centrally disposed ridge element 31. As the ridge elements 31, 34 and 35 increase in height this concentration about the center ridge element continues and is intensified, and in addition the wave energy is confined substantially entirely between the outer ridge elements 34 and 35. Because, as shown in Fig. 4C, the vectorial direction is toward the innermost portion of the central ridge element 31, and because the concentration is greatest at that point, the electric field distribution at the end of the ridge elements 31, 34 and 35 is partially transformed toward the TEM mode.

The initial surface of the unitary block 38 presented to the wave energy thus is in the form of a hemisphere, and energy is concentrated about this hemisphere and the protruding portion 32 of the ridge element 31. As the gradual change in the cross section by the sides which define the aperture 39 of the unitary block 38 is encountered, the electric field distribution fans out evenly about the center conductor 22, finally to provide a symmetrical radially distributed TEM mode. This change may be seen by comparison of Fig. 4D with Fig. 4E and then Fig. 4F;

It has beenfound that the gradual mode transformation provided by this arrangement has low reflection over a broad frequency band. It may be noted that each of the elements employed in this configuration is extremely simple to construct and install. Further, the device is compact and manufactured of readily obtainable parts, none of which are of delicate nature. Particular operative advantages are derived from the fact that the transition is accomplished along a single axis. If it is desired to utilize rotary couplers, therefore, the coaxial wavecross section.

' gradual mode distortion may be suitable.

nguide section need onlyibe matched to a like rotating "section. coupled along the same axis.

be substantially like that of Fig. 1. The unitary conductive block 38 to which the coaxial waveguide section 20 is coupled, however, may be partially within and partially without an open end of the rectangular waveguide 10. The central aperture 39 of the unitary block 38 again is of like size and configuration in cross section tothe outer conductor 21 of the coaxial waveguide section 20. The tapered surface 40 of the unitary block 38, however, may in this configuration be used to provide both the remaining two ridges of the triple ridge transition and the elliptical aperture transition to the coaxial waveguide In this example the tapered surface 40' has a central extension 41 which terminates in contact with the second broad wall 12 of the rectangular waveguide 10. The sides of the central extension 41 there- .fo're define electrical equivalents for the outer ridges 34 and 35 of the arrangement of Fig. 1.

The arrangement of Fig. 3 is perhaps more simply constructed than the arrangement of Fig. 1, because only two diiferent transition elements 31 and 41 need be fabricated. Nonetheless, the arrangement operates generally in the same way, utilizing the centrally disposed ridge element 31 to concentrate the electric field distribution toward the center conductor 22 of the coaxial section, and using the extension 41 of the tapered surface 40' of the unitary block to provide confinement of the wave energy and to complete the transformation of the mode. Both of the arrangements operate reciprocally, as is common with such waveguide transitions.

It will be understood by those skilled in the art that various other modifications of the present arrangement are feasible without material change. Wherever the mode transformation is to be completed along an axis and where both confinement and transformation of an electric field are to be employed, this technique of using It may be employed in other types of transmission lines, for example. Similiarly', the ridge sections may be varied in diiferent ways. Stepped transformer sections are sometimes employed in place of tapered sections, but the ridges maybe varied in other directions than in height.

The gradual distortion into another mode as described in the preceding sections was accomplished by a transducer made of metallic conductive parts. Mode distortions can also be achieved by a combination of metallic and dielectric elements. This fact may be better understood by a comparison of Fig. 5 and Fig. 6. Fig. 5 shows the H-field configuration within a rectangular waveguide 50 having three uniform ridge elements 51, 52 and 53. One of the ridge elements 51 is centrally placed on one broad wall of the waveguide 50, and the other two ridge elements 52 and 53 are placed on the other broad wall of the waveguide 50 and symmetrically about the center of that wall. Thus this configuration corresponds in general fashion tothe triple ridge configurations previously described, but without taper.

Fig. 6 shows the H.-field configuration for a coaxial waveguide 55 having the usual outer conductor 56 and inner conductor 57. The coaxial waveguide 55 does, however, include a dielectric element 58 of hemispherical cross section and a radius like that of the waveguide 55 extending along a length of the coaxial waveguide 55. By dielectric is meant a material having a dielectric constant sufiiciently high (of the order of two or considerably more) to effect some concentration of the energy' within the waveguide. Where the dielectric constant of the medium within the waveguide is itself high, the constant of the dielectric which is employed should of course be higher.

A comparison of the H-field configurations of Fig. 5

f6 and Fig. 6 shows similar efiiects. The efieet offthecentral ridge element 51 and the side ridge elements; 52

and 53 is to lengthen the longitudinal component of the magnetic field and to confine the transverse component. Similarly, the presence of a higher dielectric material in half a coaxial waveguide, as in Fig. 6, causes a concentration of energy within the dielectric so that the longitudinal magnetic field lines are lengthened and the circumferential component of the magnetic field is 'hemispherical. This similarity may be employed effectively to provide a different type of transition, as is shownin .Figs. 7 and 8.

Fig. 7, to which reference is now made, shows the manner in which a rectangular waveguide may be coupled to a colinear coaxial waveguide 70, with the energy transmitted being transformed between the dominant TE mode of the rectangular waveguide and the TEM mode of the coaxial waveguide. The rectangular waveguide 69 includes a central ridge element 61 tapering to a maximum height in the direction of -the coaxial waveguide 70, and a unitary conductive block 62' having .a central aperture 63 conforming to the inner circumference of the coaxial waveguide 70 The ridge 61and block 62 therefore are similar to the arrangement described above in conjunction with Fig. 3. The arrangement is like Fig, 3, however, only to the point at which the ridge element 61 couples the center conductor 72 of the coaxial waveguide 70. Positioned within the .outer conductor 71 of the coaxial waveguide is a block of dielectric material 75.

The dielectric block 75 has a minimum cross section opposite the ridge 61 of the rectangular waveguide, this cross section being in the form of an angle less than a semicircle and symmetrically abutting the ridge element 61. Proceeding in the direction away from the rectangular waveguide 66 the angle defined by the cross section of the dielectric block 75 increases until a semicircle is provided. Thus within this section of the dielectric block '75, the block 75 may be considered to be a radial segment which tapers angularly. Thereafter, in the direction away from the rectangular waveguide 60, there is a gradual taper in the level of the dielectric block 75 until the block 75 substantially fills the coaxial waveguide 70. This portion of the dielectric block may be considered to provide a planar taper.

The diagrammatic views of Fig. 8 show the sections which exist in the arrangement of Fig. 7 at various points. The cross section illustrated in Fig. 8A, for example, is an intermediate point within the rectangular waveguide. Figs. 83 and 80 show the progressive angular taper, while Fig. 8D shows the planar taper.

When constructed in accordance with Figs. 7 and 8, a

microwave transition device provides a desired coupling of energy between the rectangular waveguide 60 and the coaxial waveguide 70, together with the desired mode transformation. The increase in the cross section of the dielectric block 75, proceeding in the direction toward the coaxial waveguide 70 causes a progressive decrease in the strength of the longitudinal H-field component. Thus there is a transformation to the circular component until ultimately the circular component alone is present and the field configuration is uniform. Thus this operation corresponds in result to the transformation for the previously described exemplifications. If desired, at least a portion of this effect could be obtained by using a dielectric block '75 of more uniform cross section but gradually varying dielectric constant. The use of dielectric members may be desirable where increased power handling capabilities are preferred.

Thus there has been described an improved microwave transmission device for coupling different waveguides which are colinear. The device provides mode transformation by a gradual mode distortion technique having superior impedance matching and broadbandedness qualities. At the same time, the device is compact, simple to construct and inexpensive.

We claim: 1. A microwave transition device for interconnecting one end of a hollow waveguide wherein energy is propagated in one mode with one end of a coaxial waveguide positioned in substantial alignment with said first waveguide and adapted to have energy propagated therethrough in another mode, said transition device comprising a centrally disposed ridge element positioned against one wall of said hollow waveguide and tapering from a minimum height adjacent said wall to a height which is in registry with the center conductor of said coaxial waveguide, means disposed on an opposite wall of said hollow waveguide to define a conductive surface that extends from one of said walls to the other of said walls, said surface tapering entirely across said hollow waveguide in an opposite direction from said element, at least a portion of said surface including a central aperture matching the inner surface of the outer conductor of the coaxial waveguide and being tapered in the direction so that the center conductor of the coaxial waveguide is only gradually en- 20 are mounted on said second wall on the opposite sides of said first element to thereby define at least a portion of said conductive surface.

3. The combination of claim 1 wherein said means in- 5 cludes a block of conductive material that has one end thereof tapered to thereby form said surface, said block including a passage therethrough to form said central aperture in said surface.

1 References Cited in the file of this patent UNITED STATES PATENTS Fubini et al. July 16, 1957 Fubini July 16, 1957 Coale Aug. 13, 1957 Le Vine et al. Mar. 4, 1958 Grieg et al. Dec. 1, 1959 Robertson Ian. 26, 1960 OTHER REFERENCES Ragan: Microwave Transmission Circuits, Radiation Laboratory Series, volume 9, McGraw-Hill, 1948, pages 358-361. Copy available in Scientific Library. 

